1. Field of the Invention
The present invention relates in general to a backlighting system for liquid crystal display (LCD) panels. In particular, the present invention relates to a backlighting system for LCD panels having a structure utilizing total internal reflection to direct most of the light toward the viewing angle and to reduce energy loss, thereby increasing lighting efficiency of the system.
2. Technical Background
FIG. 1 shows a prior art backlighting system S1 for an LCD panel 1. This backlighting system includes a light source 5 providing light for the backlighting system; light pipe 3 consists of a flat front surface 9 and a flat back surface 10 in parallel with each other; one end surface 7 of the light pipe adjacent to the light source 5 receives light generated by the light source 5 to provide the backlighting for the LCD panel; the interior of the light pipe having light scattering material 8; reflective cover 6 surrounding light source 5 reflects light emitted by the light source 5 back to the light pipe 3; reflecting panel 4 reflects light exiting the light pipe through the back surface 10 back to the light pipe 3; and diffusing panel 2 between LCD panel 1 and light pipe 3 diffuses the incident light directed toward the LCD panel to provide a more uniform light source for the LCD panel and hides the dot pattern.
As shown in FIG. 1, light beam A is emitted by light source 5 and enters light pipe 3 via end surface 7. Light beam A normally travels in the longitudinal direction away from the light source 5 due to repetitive total internal reflection on the front surface 9 and back surface 10. When light beam A encounters a light scattering material 8, the condition for total internal reflection is destroyed, and its traveling path along the longitudinal direction is disrupted. A portion of the light will scatter toward the LCD panel 1 to provide the backlighting for the LCD panel. However, frequently the light beam is reflected and scattered many times before being directed toward the LCD panel 1. The more reflection and scattering the light beam experiences, the more energy loss it suffers. Thus, there are considerable drawbacks in the prior art design of this backlighting system S1 for the LCD panel.
To solve the problem of the prior art systems, a backlighting system S2, disclosed in U.S. Pat. No. 5,050,946, was designed to improve the performance of the LCD panel backlighting system S1 described above. FIGS. 2, 3, 4A and 4B together show the structure of this backlighting system S2. FIG. 2 is a partially cut-away perspective view of the backlighting system S2, FIG. 3 is an enlarged side view of the area identified by III in FIG. 2, FIG. 4A is an enlarged perspective view of the area proximate to the inclined surface N in FIG. 3, and FIG. 4B schematically shows the coordinate system used in the drawing.
Referring to both FIGS. 2 and 3 simultaneously, LCD panel backlighting system S2 also includes light source 5, reflective cover 6, light pipe 13, reflecting panel 14, and diffuser 2. The Z direction in FIG. 4B is the longitudinal direction of the light pipe 13, Y direction is the height direction of the light pipe 13, while the light tube of the light source 5 extends along the X direction, i.e., the direction perpendicular to both Y and Z directions.
It is clear from FIG. 2 and 3 that the only difference between the LCD panel backlighting system S1 and S2 is in the shape of the light pipe. In the backlighting system S2, the light pipe 13 also has a flat front surface 19, however, its back surface 20 includes a plurality of parallel sections G, E, and F, each in parallel with the front surface 19, as well as a plurality of connecting sections M, N, and so on for connecting the adjacent parallel sections. Each connecting section forms an inclined angle of approximately 135 degrees with respect to the parallel sections. The distance between the front surface 19 and the parallel sections G, E, and F decreases as the distance between the parallel sections to the end surface 17 along the longitudinal direction (the Z direction) increases (see FIG. 3).
It is also clear from FIGS. 2 and 3 that the light beam A, emitted by the light source 5 through end surface 17 into light pipe 13 with a larger incident angle with respect to the front surface 19, will experience many total internal reflections off the front surface 19 and the parallel sections G, E, F of the back surface 20, and will travel away from the light source 5. However, if light beam B, with a smaller incident angle with respect to the front surface 19, encounters one of the inclined connecting sections M or N, total internal reflection occurs, and the reflected beam B' most likely will meet the front surface 19 with an incident angle smaller than the critical angle for total internal reflection, thus exiting the light pipe from the front surface 19 and providing backlighting for fire LCD panel 1. Thus, the design of the LCD panel backlighting system S2 decreases the light path of some of the light beams traveling inside the light pipe 13, allowing those light beams to exit the light pipe from the front surface 19 after being reflected off the inclined connecting sections, thereby reducing the energy loss due to multiple reflection and scattering inside the light pipe 13.
Although this LCD panel backlighting system S2 does improve some of the drawbacks of the backlighting system S1, nevertheless, when a light beam C meets on the connecting section N at a substantially vertical angle, most of the energy of this light beam will exit the light pipe from the connecting section. Even though the reflecting panel 14 reflects most of the light back into the light pipe, the reflection coefficient is not 100%. Also, the direction of the reflected beam becomes half solid angle random reflection, that is lambertion reflection, and a large portion of energy in these light beams will be lost in the process of multiple reflection and scattering.